Re: buffer busy waits and v$filestat

Buffer busy wait has a different correlation with v$filestat and I/O. Buffer busy wait simply means that the buffer you're waiting for is pinned by somebody else.
There are 3 classic situations:

1. DBWR hasn't finished writing to the disk yet.
2. Block is locked by another node (OPS, RAC).
3. RMAN is writing the block. That's right, RMAN locks (pins) blocks in memory, otherwise it couldn't ensure consistent backup. That is the reason why RMAN doesn't need "alter tablespace begin backup" command.

To make the long story short, there is a note on metalink (Note:155971.1) with an appropriate title: Resolving Intense and "Random" Buffer Busy Wait Performance Problems. Buffer busy waits are usually a consequence of I/O subsystem not being to provide enough throughput to the database. What can you do with v$filestat? You can find where are your hot spots and fix the problem.

On 2003.07.29 17:24, Rajesh.Rao_at_jpmchase.com wrote:
> Folks,
>
> Say a session issues a read request, and finds another session already
> reading the block into the buffer cache. If this session waits N ms on a
> "buffer busy waits" event, does this N ms of wait get added to the read
> times in v$filestat? Or is the readtim in v$filestat purely physical I/O?
>
> Thanks
> Raj
>
>
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-- 
Mladen Gogala
Oracle DBA
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Author: Mladen Gogala
  INET: mgogala_at_adelphia.net

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